Normally white LCD including first and second biaxial retarders

ABSTRACT

A normally white liquid crystal display is provided with a positively birefringent uniaxial retardation film having a retardation value of from about 100-200 nm. The retardation film is provided on one side of the liquid crystal layer, the liquid crystal being sandwiched between a pair of orientation or buffing films which orient the liquid crystal molecules adjacent thereto in predetermined directions. The optical axis of the retardation film is rotated from about 2°-20°, most preferably from about 6°-10° relative to the buffing direction on the opposite side of the liquid crystal layer. This rotation of the retardation film optical axis allows for the high contrast ratio viewing zone of the display to be shifted vertically into either the positive or negative vertical viewing region depending upon the direction of rotation of the retardation film optical axis. Alternatively, biaxial retardation films having similar retardation values may be utilizied according to the teachings of this invention.

RELATED APPLICATIONS

This is a continuation of U.S. Ser. No. 08/747,671, filed Nov. 12, 1996(U.S. Pat. No. 5,737,048); which is a continuation of Ser. No.08/255,971, filed Jun. 8, 1994 (U.S. Pat. No. 5,576,861); which is a CIPof Ser. No. 08/235,691, filed Apr. 29, 1994 (U.S. Pat. No. 5,594,568);and a CIP of Ser. No. 08/167,652, filed Dec. 15, 1993 (U.S. Pat. No.5,570,214), the entire disclosures of which are hereby incorporatedherein by reference.

This invention relates to a liquid crystal display having at least oneretardation film. More particularly, this invention relates to anormally white liquid crystal display including a retardation filmdisposed on one side of the liquid crystal layer, the optical axis ofthe retardation film being oriented according to the manufacturer'sdesired specification.

BACKGROUND OF THE INVENTION

Liquid crystal materials are useful for electronic displays becauselight traveling through a layer of liquid crystal (LC) material isaffected by the anisotropic or birefringent value (Δn) of the LCmaterial which in turn can be controlled by the application of a voltageacross the LC. Liquid crystal displays (LCDs) are commonly used inapplications such as avionic cockpit displays, portable computers,calculators, etc.

Informational data in typical liquid crystal displays is presented inthe form of a matrix array of rows and columns of numerals or characterswhich are generated by a number of segmented electrodes arranged in amatrix pattern. The segments are connected by individual leads todriving electronics which apply a voltage to the appropriate combinationof segments and adjacent LC material in order to display the desireddata and/or information by controlling the light transmitted through theliquid crystal material.

Contrast ratio is one of the most important attributes considered indetermining the quality of both normally white (NW) and normally black(NB) liquid crystal displays. The contrast ratio in a normally whitedisplay is determined in low ambient conditions by dividing the“off-state” light transmission (high intensity white light) by the“on-state” or darkened transmitted intensity. For example, if the“off-state” transmission is 200 fL at a particular viewing angle and the“on-state” transmission is 5 fL at the same viewing angle, then thedisplay's contrast ratio at that particular viewing angle is 40 or 40:1for the particular “on-state” driving voltage utilized.

Accordingly, in normally white LCDs the primary factor adverselylimiting the contrast ratio is the amount of light which leaks throughthe display in the darkened or “on-state”. In a similar manner, innormally black displays, the primary factor limiting the contrast ratioachievable is the amount of light which leaks through the display in thedarkened or “off-state”. The higher and more uniform the contrast ratioof a particular display over a wide range of viewing angles, the betterthe LCD.

Normally black (NB) twisted nematic displays typically have bettercontrast ratio contour curves or characteristics then do theircounterpart NW displays in that the NB image can be seen better at largeviewing angles. However, NB displays are much harder to manufacture thanNW displays due to their high dependence on the cell gap or thickness“d” of the liquid crystal layer as well as on the temperature of theliquid crystal material itself. Accordingly, a long-felt need in the arthas been the ability to construct a normally white display with highcontrast ratios over a large range of viewing angles, rather than havingto resort to the more difficult to manufacture NB display to achievethese characteristics.

What is generally needed in NW displays is an optical compensating orretarding element(s), i.e. retardation film, which introduces a phasedelay that restores the original polarization state of the light, thusallowing the light to be substantially blocked by the output polarizerin the “on-state”. Optical compensating elements or retarders are knownin the art and are disclosed, for example, in U.S. Pat. Nos. 5,184,236,5,196,953, 5,138,474, and 5,071,997, the disclosures of which are herebyincorporated herein by reference.

FIG. 1 is a contrast ratio curve graph for a prior art normally whitetwisted nematic light valve including a rear linear polarizer having atransmission axis oriented in a first direction, a front or light exitlinear polarizer having a transmission axis defining a second directionwherein the first and second directions are substantially perpendicularto one another, a liquid crystal material having a cell gap “d” of about5.86 μm and a birefringence (Δn) of about 0.084 at room temperature, arear buffing or orientation film buffed in the second direction, and afront orientation film buffed in the first direction. The temperature atwhich FIG. 1 was developed was about 34.4° C. This light valve did notinclude a retarder.

The contrast ratio curves of FIG. 1 were plotted utilizing a 6.8 volt“on-state” driving voltage, a 0.2 volt “off-state” or V_(OFF) voltage,and by conventionally backlighting the display with white light. As canbe seen in FIG. 1, the viewing zone or envelope of the light valve whilebeing fairly broad horizontally in the lower vertical region becomesnarrowed or constricted in, the positive vertical viewing region. Forexample, at positive 20° vertical, the 10:1 and greater contrast ratioregion extends horizontally over only a total of about 70° while at −20°vertical, this same 10:1 contrast ratio zone extends over a horizontaltotal of about 100°. Therefore, because of the non-uniform or skewedshape of the viewing zone or envelope shown in FIG. 1, it is evidentthat viewers in the positive vertical viewing region will havedifficulty viewing displayed images at medium and large horizontalviewing angles such as about ±40°. This graph is illustrative of thecommon problems associated with typical normally white liquid crystaldisplays in that their contrast ratios are limited at increasedhorizontal and vertical viewing angles.

FIG. 2 is a driving voltage versus intensity (fL) plot of the prior artlight valve described above with respect to FIG. 1, this plotillustrating the gray level behavior of this light valve. The variouscurves represent horizontal viewing angles from about −60° to +60° alongthe 0° vertical viewing axis.

Gray level performance and the corresponding amount of inversion areimportant in determining the quality of an LCD. Conventional liquidcrystal displays typically utilize anywhere from about 8 to 64 differentdriving voltages. These different driving voltages are generallyreferred to as “gray level” voltages. The intensity of light transmittedthrough the pixel(s) or display depends upon the driving voltageutilized. Accordingly, conventional gray level voltages are used togenerate dissimilar shades of color so as to create different colorswhen, for example, the shades are mixed with one another.

Preferably, the higher the driving voltage in a normally white display,the lower the intensity (fL) of light transmitted therethrough. Likewisethen, the lower the driving voltage, the higher the intensity of lightreaching the viewer. The opposite is true in normally black displays.Thus, by utilizing multiple gray level driving voltages, one canmanipulate either a NW or NB liquid crystal display to emit desiredintensities and shades of light/color. A gray level V_(ON) is generallyknown as any driving voltage greater than V_(th) (threshold voltage) upto about 5-6.5 volts.

Gray level intensity in LCDs is dependent upon the display's drivingvoltage. It is desireable in NW displays to have an intensity versusdriving voltage curve wherein the intensity of light emitted from thedisplay or pixel continually and monotonically decreases as the drivingvoltage increases. In other words, it is desireable to have gray levelperformance in an NW pixel such that the intensity (fL) at 6.0 volts isless than that at 5.0 volts, which is in turn less than that at 4.0volts, which is less than that at 3.0 volts, which is in turn less thanthat at 2.0 volts, etc. Such desired gray level curves across wideranges of view allow the intensity of light reaching viewers atdifferent viewing angles via the pixel(s) or display to be easily andconsistently controlled.

Turning again to FIG. 2, the intensity versus driving voltage curvesillustrated therein of the FIG. 1 light valve having no retardation filmare undesireable because of the inversion humps present in the areas ofthe curves having driving voltages greater than about 3 or 3.2 volts.The intensity aspect of the curves monotonically decreases as thedriving voltage increases in the range of from about 1.6-3.0 volts, butat a driving voltage of about 3.2 volts, the intensities at a pluralityof viewing angles begin to rise as the voltage increases from about 3.2volts up to about 6.8 volts. Such rises in intensity as the drivingvoltage increases are known as “inversion humps”. Inversion humps leadto the display or light valve emitting different colors via the samepixel at different viewing angles for the same driving voltage. Clearly,this is undesirable. Whilte the inversion humps of FIG. 2 include onlyrise portions, inversion humps often include both rise and fall portionsas will be appreciated by those of ordinary skill in the art, thusenabling the “inversion humps” to actually look like humps.

A theoretically perfect driving voltage versus intensity (fL) curve foran NW display would have a decreased intensity (fL) for each increase ingray level driving voltage at all viewing angles. In contrast to this,the inversion humps of FIG. 2 represent large increases in intensity ofradiation emitted from the light valve for each corresponding increasein gray level driving voltage above about 3.2 volts. Accordingly, itwould satisfy a long-felt need in the art if a normally white liquidcrystal display could be provided with no or little inversion.

U.S. Pat. No. 5,184,236 discloses an NW display including a pair ofretardation films provided on one side of the LC layer, theseretardation films having retardation values of about 300 nm or greater.The viewing characteristics of the LCDs of this patent could be improvedupon with respect to contrast ratio, inversion, and uniformity as wellas the position of the viewing zone by utilizing retarders of differentvalues and orientations. Furthermore, it is felt that such improvementsmay be achieved with a reduced number of retardation films thus reducingthe cost and complexity of the display.

The parents of this application, i.e. Ser. Nos. 08/167,652 and08/235,691 incorporated herein by reference, provide for NW displayswith a pair of retardation films having retardation values of about80-200 nm. While the different embodiments of Ser. No. 08/167,652 and08/235,691 provide excellent results with respect to viewingcharacteristics, the disclosure of this application allows improvedviewing characteristics in the vertical viewing regions whilesacrificing certain viewing characteristics at other viewing angles.

FIG. 3 illustrates the angular relationships between the horizontal andvertical viewing axes and angles described herein relative to a liquidcrystal display and conventional LCD angles φ and Θ. The +X, +Y, and +Zaxes shown in FIG. 3 are also defined in other figures herein.Furthermore, the “horizontal viewing angles” (or X_(ANG)) and “verticalviewing angles” (or Y_(ANG)) illustrated and described herein may betransformed to conventional LCD angles: azimuthal angle φ; and polarangle Θ, by the following equations:

Tan(X _(ANG))=Cosine(φ)·Tan(Θ)

Sine(Y _(ANG))=Sine(Θ)·Sine(φ)

or

Cosine(Θ)=Cosine(Y _(ANG))·Cosine(X _(ANG))

Tan(φ)=Tan(Y _(ANG))÷Sine(X _(ANG))

The term “rear” when used herein but only as it is used to describesubstrates, polarizers, electrodes, buffing zones, and orientation filmsmeans that the described element is on the backlight side of the liquidcrystal material, or in other words, on the side of the LC materialopposite the viewer.

The term “front” when used herein but only as it is used to describesubstrates, polarizers, electrodes, buffing zones and orientation filmsmeans that the described element is located on the viewer side of theliquid crystal material.

The LCDs and light valves herein include a liquid crystal material witha birefringence (Δn) of 0.084 at room temperature, Model No. ZLI-4718obtained from Merck.

The term “retardation value” as used herein means “d·Δn” of theretardation film or plate, wherein “d” is the film thickness and “Δn” isthe film birefringence.

The term “interior” when used herein to describe a surface or side ofelements (or an element itself), means the side or surface closest tothe liquid crystal material.

The term “light valve” as used herein means a liquid crystal displayincluding a rear linear polarizer, a rear transparent substrate, a rearcontinuous pixel electrode, a rear orientation film, an LC layer, afront orientation film, a front continuous pixel electrode, a frontsubstrate, and a front polarizer (without the presence of color filtersand driving active matrix circuitry such as TFTs). Such a light valvemay also include a retardation film(s) disposed on either side of the LClayer as described with respect to each example and embodiment herein.In other words, a “light valve” may be referred to as one giant pixelwithout segmented electrodes.

It is apparent from the above that there exists a need in the art for anormally white liquid crystal display wherein the viewing zone of thedisplay has both high contrast ratios and little or no inversion over awide range of viewing angles, the viewing zones position being shiftableto different vertical regions so as to allow viewers at suchpredetermined viewing angles (e.g. positive vertical viewing angles) tobe able to satisfactorily view the displayed image.

This invention will now be described with respect to certain embodimentsthereof, accompanied by certain illustrations wherein:

SUMMARY OF THE INVENTION

Generally speaking this invention fulfills the above-described needs inthe art by providing a method of shifting the high contrast ratioviewing zone of a twisted nematic normally white liquid crystal displayupward into the positive or upward vertical viewing zone, the methodcomprising the steps of:

a) sandwiching a twisted nematic liquid crystal layer between a pair ofelectrodes, the liquid crystal layer having a thickness of from about4.5-6.5 μm;

b) orienting the liquid crystal molecules on a first side of the liquidcrystal layer in a first direction;

c) orienting the liquid crystal molecules on a second side of the liquidcrystal layer in a second direction, the first and second directionsbeing different from one another in a manner such that the liquidcrystal layer when in the off-state twists at least one visiblewavelength of light less than about 100°;

d) providing a retardation film having a retardation value “d·Δn” in therange of about 100-250 nm on the first side of the liquid crystal layer,wherein “d” is the thickness of the retardation film and “Δn” is itsbirefringence;

e) rotating the optical axis of the retardation film from above 2°-20°relative to the second direction, the rotating of the optical axisshifting the high contrast viewing zone vertically so that viewers atsuch viewing angles may see a high contrast image with reducedinversion.

This invention further fulfills the above-described needs in the art byproviding a twisted nematic liquid crystal display capable of displayingan image to a viewer, the display comprising:

a pair of electrodes sandwiching a twisted nematic liquid crystal layertherebetween, the pair of electrodes for applying a voltage across theliquid crystal layer;

first and second orientation means disposed adjacent the liquid crystallayer on opposite sides thereof, the first orientation means defining afirst orientation or buffing direction and the second orientation meansdefining a second orientation or buffing direction, the first and secondorientation directions for aligning the liquid crystal molecules of theliquid crystal layer in a predetermined manner;

a positively birefringent unaxial retardation film having a retardationvalue “d·Δn” in the range of about 100-200 nm, where “d” is thethickness of the retardation film and “Δn” is its birefringent value,wherein the retardation film is disposed on the same side of the liquidcrystal layer as the first orientation means, the retardation film beingoriented such that its optical axis is substantially parallel ±about 20°to the second orientation or buffing direction of the second orientationmeans thereby enabling the liquid crystal display to display to theviewer an image with improved contrast ratios and reduced inversion.

This invention further fulfills the above-described needs in the art byproviding a method of making a normally white twisted nematic liquidcrystal display, a method comprising the steps of:

a) sandwiching a liquid crystal layer between first and secondelectrodes, the liquid crystal layer having a thickness “d” of fromabout 4.5 to 6.5 μm;

b) providing a first orientation means between the first electrode andthe liquid crystal layer, the first orientation means for orienting LCmolecules of the LC layer in a first direction adjacent the firstorientation means;

c) providing a second orientation means between the LC layer and thesecond electrode, the second orientation means for orienting LCmolecules in a second direction adjacent the second orientation means;

d) disposing a positively birefringent uniaxial retardation film on thesame side of the LC layer as the first electrode and first orientationmeans, the retardation film having a retardation value of from about100-200 nm; and

e) orientating the optical axis of the uniaxial retardation filmsubstantially parallel ± about 20° to the second direction defined bythe second orientation means whereby the normally white display exhibitsimproved contrast ratios and reduced inversion.

This invention will now be described with respect to certain embodimentsthereof, wherein:

IN THE DRAWINGS

FIG. 1 is a contrast ratio plot of a prior art light valve whichutilized white light and an “on-state” driving voltage of about 6.8volts.

FIG. 2 is an intensity versus driving voltage plot of the prior artlight valve of FIG. 1, this plot illustrating a fairly large amount ofundesireable inversion over a wide range of horizontal viewing angles atdriving voltages greater than about 3 volts.

FIG. 3 is a graph illustrating the angular relationship between thehorizontal and vertical viewing angles discussed herein, and theirrelationship with conventional liquid crystal display viewing angles:azimuthal angle φ; and polar angle Θ.

FIG. 4 is an exploded perspective schematical diagram of the opticalcomponents and their respective orientations of an LCD according to afirst embodiment of this invention.

FIG. 5 is a top view illustrating the optical component angularrelationships of the liquid crystal display of FIG. 4.

FIG. 6 is a side elevational cross-sectional view of the LCD of thefirst or FIGS. 4-5 embodiment of this invention.

FIG. 7 is an exploded perspective schematical diagram of the opticalcomponents and their respective orientations of an LCD according to asecond embodiment of this invention, this embodiment being “P-buffed” asopposed to the “X-buffed” first embodiment.

FIG. 8 is an exploded perspective schematical diagram of the opticalcomponents and their respective orientations of an LCD according to athird embodiment of this invention wherein the retardation film isdisposed on the rear or backlight side of the liquid crystal layer.

FIG. 9 is a white light contrast ratio contour plot of the normallywhite Display “A” of Example 1 when a driving voltage of about 6.8 voltswas applied.

FIG. 10 is a white light contrast ratio contour plot of the normallywhite Display “A” of Example 1 when utilizing a driving voltage of about6.0 volts was applied.

FIG. 11 is a white light transmission (fL) versus driving voltage plotof the normally white Display “A” of Example 1, this plot illustratingthe viewing characteristics at a plurality of horizontal viewing anglesdisposed along the 0° vertical viewing axis.

FIG. 12(a) is a white light contrast ratio contour plot of thecomparative normally white Display “B” of Example 1 when a drivingvoltage of about 6.8 volts was applied.

FIG. 12(b) is a white light contrast ratio contour plot of thecomparative NW Display “C” of Example 1 when a driving voltage of about6 volts was applied.

FIG. 13 is a white light contrast ratio contour plot of the NW lightvalve of Example 2 when a driving voltage of about 5.0 volts wasapplied.

FIG. 14 is a white light contrast ratio contour plot of the NW lightvalve of Example 2 when a driving voltage of about 4.0 volts wasapplied.

FIG. 15 is a white light contrast ratio contour plot of the normallywhite AMLCD of Example 3 when a driving voltage of about 6.0 volts wasapplied.

FIG. 16 is an exploded perspective schematical view of the opticalcomponents and their respective orientations of another embodiment ofthis invention wherein first and second retardation films (uniaxial orbiaxial) are disposed on one side of the liquid crystal layer, asdisclosed in U.S. Pat. No. 5,594,568 (Ser. No. 235,691), which wasincorporated herein by reference above.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OF THIS INVENTION

Referring now more particularly to the accompanying drawings in whichlike reference numerals indicate like parts.

FIG. 4 is an exploded schematic view of the optical components and theirrespective orientations of an LCD according to a first embodiment ofthis invention, the LCD being an AMLCD having a matrix array of pixelsand colored subpixels in certain embodiments. As shown, this display (ordisplay assembly) includes from the rear forward toward viewer 1,conventional backlight 3, rear or light-entrance linear polarizer 5,rear buffing or orientation film 7, liquid crystal layer 9, frontbuffing or orientation film 11, retardation film 13, and finally frontor light-exit linear polarizer 15.

Backlight 3 is conventional in nature and emits substantially collimatedor alternatively diffused light toward the display panel including rearpolarizer 5 in certain embodiments of this invention. Backlight 3 maybe, for example, the backlighting assembly disclosed in commonly ownedU.S. Pat. No. 5,161,041, the disclosure of which is hereby incorporatedherein by reference. Other conventional high intensity substantiallycollimated backlight assemblies may also be used.

Rear and front polarizers 5 and 15 are linear in nature in certainembodiments of this invention and their respective linear transmissionaxes P_(R) and P_(F) are orientated such that the displays of thedifferent embodiments are of the normally white (NW) type. Therefore,when a driving voltage below the threshold voltage V_(th) is appliedacross liquid crystal layer 9, transmission axes P_(R) and P_(F) ofpolarizers 5 and 15 respectively are orientated such that the lightemitted from backlight 3 proceeds through and is linearly polarized indirection P_(R) by polarizer 5, is then twisted (e.g. about 80°-100°) byLC material 9, and finally exits polarizer 15 via transmission axisP_(F) thus reaching viewer 1. The light reaches viewer 1 because itspolarization directon upon reaching front polarizer 15 is similar tothat of axis P_(F). Thus, a NW display or pixel to which a voltage lessthan V_(th) is applied is said to be in the “off-state” and appearswhite (or colored if colored filters are present) to the viewer.

However, when a substantial driving voltage (e.g. about 6 volts) isapplied across selected NW pixels of the matrix array including liquidcrystal layer 9, the light transmitted through rear polarizer 5 is notsignificantly twisted by LC layer 9 and thus is substantially blocked byfront polarizer 15 due to the fact that the polarization direction oflight reaching the interior surface of front polarizer 15 issubstantially perpendicular to transmission axis P_(F) thereby resultingin substantially no light reaching viewer 1 by way of the selectedpixels to which the substantial driving voltage is applied. Thus, theselected pixels driven in the matrix array appear darkened to viewer 1,these pixels said to be in the “on-state”. As will be appreciated bythose of skill in the art, the amount of light reaching viewer 1 isdictated by the voltage applied to LC layer 9—the higher the drivingvoltage, the darker the selected driven pixel(s) appear.

In certain embodiments of this invention, transmission axis P_(R) ofrear polarizer 5 and transmission axis P_(F) of front polarizer 15 areoriented in a manner substantially perpendicular to one another so as todefine a normally white twisted nematic cell. However, polarizers 5 and15 may be oriented in other conventional manners which also allow thecell or display to be of the normally white type.

Rear and front orientation or buffing films 7 and 11, respectively, areconventional and made of a substantially transparent polyimide incertain embodiments of this invention. Rear orientation film 7 isconventionally buffed or oriented in direction B_(R) as shown in FIG. 4.Likewise, front film 11 is conventionally buffed in direction B_(F).Buffing directions B_(R) and B_(F) are oriented substantiallyperpendicular to one another in certain embodiments of this invention soas to allow the molecules of liquid crystal layer 9 when in the off ornon-driven state to be twisted from about 80°-100°, most preferablyabout 90°. The term “off-state” means that a voltage below the thresholdvoltage (V_(th)) is applied across LC layer 9.

Due to the orientation of buffing directions B_(R) and B_(F) oforientation films 7 and 11 respectively, the polarization direction ofnormally incident light emitted from backlight 3 reaching liquid crystalmaterial 9 is twisted in a conventional manner by the liquid crystalmolecules as it passes through layer 9, when, of course, the display (orselected pixels thereof) is in the off-state.

However, when a substantially full driving voltage, e.g. about 6 voltsor above, is applied to liquid crystal layer 9 (or selected pixelsthereof to form the intended image), the normally incident light frombacklight 3 reaching layer 9 is permitted to pass therethrough whilesubstantially maintaining its initial direction of polarization. This isdue to the fact that when a voltage is applied across LC material 9, theLC molecules are caused to become substantially aligned with one anotherin the vertical direction as shown in FIG. 4. Therefore, little orsubstantially no twisting occurs when such a driving voltage (e.g. about6 volts) is applied and thus the direction of polarization of lightpassing through layer 9 is substantially maintained.

The voltage amount applied across LC layer 9 determines the degree oftwisting of the liquid crystal molecules and thus dictates thepolarization direction of light emitted from the front or viewer side oflayer 9. In turn, the polarization direction of light reaching polarizer15 dictates the amount of light permitted to pass therethrough via axisP_(F) and reach viewer 1 in that the closer aligned transmission axisP_(F) and the polarization direction of light reaching polarizer 15, themore light which is allowed to pass and reach viewer 1.

While the application of voltage>V_(th) to layer 9 causes the LCmolecule to substantially align vertically, the LC molecules nevercompletely stand on end or become perfectly aligned in the verticaldirection as is known in the art. This gives rise to the need forretardation film(s).

Retardation film 13 in this first embodiment is disposed on the viewerside of liquid crystal layer 9 thereby being sandwiched between frontpolarizer 15 and front orientation film 11. Surprisingly, it has beenfound that the provision of retardation film 13 on a single side oftwisted nematic LC layer 9 reduces inversion and improves viewing zonecontrast ratios at large viewing angles when the retardation value ofthe film is reduced relative to the prior art to retardation values offrom about 100-200 mm.

Retardation film 13 in certain embodiments of this invention ispositively birefringent and uniaxial in nature, this film being obtainedfrom, for example, Nitto Corporation, Japan, or Nitto Denko America,Incorporated, New Brunswick, N.J. as Model No. NRF-140 (140 nmretarder).

Alternatively, it is believed that biaxial retardation films havingsimilar retardation values may also provide excellent results, suchbiaxial retardation films and values being disclosed in aforementionedU.S. Ser. No. 08/235,691 filed Apr. 29, 1994.

With reference to FIGS. 4-5, axis P_(R) and direction B_(F) aresubstantially parallel to one another in certain embodiments of thisinvention while direction B_(R), axis P_(F), and direction R (or R₀) arealso substantially parallel ±about 5° to one another. Accordingly, insuch embodiments, axis P_(R) and direction B_(R) are substantiallyperpendicular to one another as are axis P_(F) and direction B_(F). Adisplay having such an optical arrangement is said to be “X-buffed”. Theterm “X-buffed” means that rear polarizer axis P_(R) is substantiallyperpendicular to rear buffing direction B_(R) while front polarizer axisP_(F) is substantially perpendicular to front buffing direction B_(F).Thus, the first embodiment of this invention illustrated in FIGS. 4-6 isan LCD of the “X-buffed” type.

Alternatively, an LCD may be “P-buffed” instead of “X-buffed” in certainembodiments, “P-buffed” meaning that rear polarizer axis P_(R) issubstantially parallel to rear buffing direction B_(R) and frontpolarizer axis P_(F) is substantially parallel to front buffingdirection B_(F).

Optical axis R of retardation film 13 in the first embodiment of thisinvention (see FIGS. 4-6) may be aligned in direction R₀ so as to besubstantially parallel to axis P_(F) and buffing direction B_(R).Alternatively, optical axis R of retardation film 13 may be rotatedeither clockwise or counterclockwise relative to directions R₀ andB_(R).

The effect of rotating optical axis R of film 13 relative to directionR₀ is to shift the viewing zone or envelope of the display verticallyinto either the upper or lower vertical region as will be furtherdiscussed in the examples below. When film 13 is disposed forward of LClayer 9 and optical axis R of retardation film 13 is rotated clockwiserelative to direction R₀ (as shown in FIGS. 4-5) so as to define angle Θtherebetween, the high contrast viewing envelope of the display isshifted into the upper or positive vertical region so as to provideviewer 1 with a high contrast ratio image at increased positive verticalviewing angles. To achieve such a high quality shifted image in thepositive vertical region, optical axis R of retardation film 13 isrotated clockwise (to define Θ) from about 2°-20° relative to R₀, morepreferably about 4°-15°, and most preferably from about 6°-10° incertain embodiments of this invention. The term “clockwise” is definedas being viewed from the position of viewer 1 in FIG. 4 (or as shown inFIGS. 4-5).

Alternatively, optical axis R of film 13 may be rotated counterclockwiserelative to direction R₀ so as to shift the high contrast viewingenvelope of the display into the negative vertical viewing region whenfilm 13 is positioned forward of LC layer 9. The same degrees ofrotation discussed above relative to clockwise rotation of axis R alsoapply to this alternative counterclockwise rotation of optical axis Rrelative to directions R₀ and B_(R).

The ability to shift the viewing zone vertically via rotation of film 13is advantageous in that it allows for excellent positive or negativevertical viewing charactertistics in situations where they are needed.Thus, if a customer desires good positive vertical viewing, themanufacturer need simply rotate optical axis R of retardation film 13 inthe clockwise direction as discussed above.

The retardation value “d·Δn” of retardation film 13 is a criticalparameter in achieving the surprising results of the differentembodiments of this invention, where “d” is the thickness of theretardation film and “Δn” is its birefringent value. In certainembodiments, retardation film 13 is of the uniaxial positivelybirefringent type and has a retardation value of from about 100-200 nm,more preferably from about 110-180 nm, and most preferably from about120-160 nm. The biaxial retardation values of the biaxial retardersdisclosed in Ser. No. 08/235,691 will also suffice in certainembodiments. In certain embodiments of this invention, as disclosed inU.S. Pat. No. 5,594,568 (i.e. Ser. No. 08/235,691) which wasincorporated into this application by reference on page 1 of thisapplication, first and second biaxial negative retarders, each having aretardation value d·Δn_(zy) of from about −10 to −100 nm, may beprovided for improving the viewing characteristics of the display. Incertain embodiments, the first and second negative biaxial retardationfilms or layers may be provided on the same side of the liquid crystallayer.

FIG. 6 is a side elevational cross-sectional view of the NW liquidcrystal display of the first embodiment of this invention. As shown, thedisplay includes the optical elements illustrated in FIGS. 4-5 as wellas rear transparent substrate 17, front transparent substrate 19, rearelectrode 21, and front electrode 23.

Transparent substrates 17 and 19 are made of glass or transparentplastic in certain embodiments of this invention, rear substrate 17being sandwiched between rear polarizer 5 and rear electrode 21 andfront transparent substrate 19 being disposed between front electrode 23and retardation film 13. Alternatively, retardation film 13 may bedisposed interior substrate 19 as opposed to its exterior position shownin FIG. 6.

Rear and front electrodes 21 and 23 are conventional in nature and madeof transparent ITO in certain embodiments of this invention. Whileelectrodes 21 and 23 are both shown in FIG. 6 as being continuous innature, rear electrode 21 in AMLCD applications may be conventionallysegmented into a number of different pixel or colored subpixelelectrodes. In such AMLCDs, each pixel or colored subpixel may beindividually addressed via a corresponding conventional a-Si TFT ordiode.

For example, electrode 21 may be divided into thirty separate andindependent subpixel electrodes, ten of which are associated withcorresponding blue filters (not shown) so as to define blue subpixels,another ten of which are associated with corresponding red filters (notshown) thereby defining red subpixels, and the remaining ten beingassociated with green color filters (not shown) so as to define greensubpixels. The color filters (not shown) are disposed on the opposingside of LC layer 9 with respect to the segmented electrodes. In such anarrangement, the thirty subpixels may make up ten pixels, each pixelhaving a red, green, and blue subpixel therein arranged in a triangularfashion in certain embodiments.

With reference to FIGS. 4-6, in a typical operation of this firstembodiment, the display operates as follows. White light is firstemitted from conventional collimating backlight 3 and directed towardthe rear side of the display panel. The light from backlight 3 hits rearpolarizer 5 and is linearly polarized in accordance with polarizationaxes P_(R). After being initially polarized, the linearly polarizedlight proceeds through rear transparent substrate 17, rear electrode(s)21, and rear buffing or orientation film 7 before reaching liquidcrystal layer 9.

When liquid crystal layer 9 is in the off-state, the light proceedingtherethrough is twisted (preferably about In certain 90°) before exitinglayer 9 and reaching front buffing film 11. However, when LC layer 9 hasa voltage (e.g. about 6 volts) above V_(th) applied thereto and istherefore in the on-state, the polarization direction of the lightreaching its rear surface remains substantially unchanged as it proceedsthrough layer 9 and exits its front surface adjacent front orientationfilm 11 because the application of voltage across layer 9 causes the LCmolecules thereof to become substantially aligned vertically or “standup” as known in the art. Accordingly, the polarization direction of thelight exiting LC layer 9 depends upon the voltage applied across theliquid crystal material—the higher the voltage, the more the LCmolecules become aligned and the less twisting which occurs.

After exiting the front or exit side of liquid crystal layer 9, thelight proceeds through front orientation film 11, front transparent ITOelectrode 23, subpixel color filters (not shown) if present, and fronttransparent substrate 19 before reaching uniaxial retardation film 13.As the light proceeds through retardation film 13, the filmconventionally introduces a phase delay that substantially restores theoriginal polarization state of the light to what it was before itentered liquid crystal layer 9 (assuming the display is in the“on-state”).

A need for retardation film 13 arises because when a driving voltage isapplied across LC layer 9, the liquid crystal molecules become alignedvertically, but never completely. In other words, the liquid crystalmolecules, even when a high driving voltage is applied thereto, aretilted slightly from the vertical. This inevitable tilting of the LCmolecules creates the need for retardation film 13 which in effectproduces a phase delay which reverses the effect caused by thenon-perfect vertical alignment of the LC molecules.

After exiting retardation film 13, the light which originated frombacklight 3 reaches the interior side of front linear polarizer 15. Asdiscussed above, the polarization direction of the light reaching frontpolarizer 15 depends upon the driving voltage (or absence thereof)applied across liquid crystal layer 9. Thus, with respect to LC pixelsof the matrix array in the off-state, the polarization direction oflight reaching front polarizer 15 is substantially aligned withtransmission axis P_(F) which results in these off-state pixelsappearing white or colored to the viewer.

However, with respect to on-state pixels in which a driving voltage>V_(th) is applied across LC material 9, the polarization direction oflight reaching front polarizer 15 is not substantially aligned withtransmission axis P_(F) thus resulting in on-state pixels appearingdarkened to viewer 1 because polarizer 15 substantially blocks the lightfrom reaching viewer 1. In such a manner, the application ofpredetermined driving voltages to selective pixels or colored subpixelsresults in desired images being displayed to viewer 1.

FIG. 7 is an exploded schematic view of the optical components and theirrespective orientations of an LCD according to a second embodiment ofthis invention. This second embodiment depicted in FIG. 7 differs fromthe first embodiment (FIGS. 4-6) in that the first embodiment is“X-buffed” and this second embodiment is “P-buffed”. In other words,transmission axis P_(R) of rear linear polarizer 5 in this embodiment issubstantially parallel to buffing direction B_(R) of rear orientationfilm 7, and transmission axis P_(F) of front polarizer 15 issubstantially parallel to buffing direction B_(F) of front orientationfilm 11, thus defining a “P-buffed” display.

As will be appreciated by those of skill in the art, the display of thefirst embodiment may be adjusted so as to be transformed into the LCD ofthe second embodiment simply by rotating rear and front polarizers 5 and15 respectively about 90° each, the rest of the cell such as LC layer 9,orientation films 7 and 11, retardation film 13, substrates 17 and 19,and electrodes 21 and 23 remaining substantially the same in both thefirst and second embodiments.

FIG. 8 is an exploded schematic view of the optical components and theirrespective orientations of an “X-buffed” LCD according to a thirdembodiment of this invention. While both the first and third embodimentsillustrated and described herein are “X-buffed”, the principaldifference therebetween is the position of retardation film 13. As shownin FIG. 8, retardation film 13 is disposed rearward or on the backlightside of liquid crystal layer 9 as opposed to its disposition on thefront side thereof in the first embodiment of this invention.

A significant advantage associated with the positioning of retardationfilm 13 rearward of liquid crystal layer 9 is the reduction of ambientlight reflection off of the front of the display panel, this reductionbeing attributed to fewer mismatching indices of refraction forward ofliquid crystal layer 9 as discussed in aforesaid Ser. No. 08/235,691.

With respect to the third embodiment shown in FIG. 8, optical axis R ofretardation film 13 may be rotated clockwise or counterclockwiserelative to direction R₀ and buffing direction B_(F), counterclockwiserotation causing the viewing envelope to shift into the upper orpositive vertical region as in the first embodiment and clockwiserotation causing the envelope to shift into the negative or lowervertical viewing region. Therefore, if film 13 is disposed on the viewerside of liquid crystal layer 9, it must be rotated clockwise in order toshift the viewing envelope into the upper or positive vertical region,while if retardation film 13 is disposed rearward of liquid crystallayer 9 as in FIG. 8, counterclockwise rotation of optical axis Rrelative to directions R₀ and B_(F) as shown in FIG. 8 will cause theviewing envelope to shift into the upper vertical region.

With respect to the optical components of the third embodiment,transmission axis P_(R) of rear polarizer 5 is substantially parallel todirection R₀ and buffing direction B_(F). Likewise, transmission axisP_(F) of front polarizer 15 is substantially parallel to buffingdirection B_(R) of rear orientation film 7, buffing directions B_(F) andB_(R) being substantially perpendicular to one another. With respect tothe retardation value of retarder 13, each of the first, second, andthird embodiments utilize the aforediscussed retardation values.

This invention will now be described with respect to certain examples asfollows:

EXAMPLE 1

In this first Example, three separate normally white a-Si TFT driventwisted nematic AMLCDs were manufactured and tested for purposes ofcomparison. The three AMLCDs are referred to in this Example as Display“A”, Display “B”, and Display “C” respectively. Each of the three AMLCDsof this Example utilized the same liquid crystal layer, RGB colorfilters, orientation films, electrodes, and transparent substrates. Inother words, Displays “B” and “C” were constructed simply by adjustingor replacing the polarizers and/or retardation film 13.

The liquid crystal material of each display had a birefringence (ΔN) ofabout 0.084 at room temperature and was obtained from E. Merck Ltd. orits United States representative E.M. Industries, Inc., Hawthorne, N.Y.as Model No. ZLI-4718. Each of the three displays was tested at about35°-45° C. The electrodes were conventional in nature and made oftransparent ITO, the substrates were made of glass, and the buffing ororientation films were conventional in nature and made of a polyimidematerial. All three NW AMLCDs of this Example were of the RGB coloredtype and had red cell gaps “d” of about 5.6 μm, and green and blue cellgaps of about 5.3 μm, each pixel having a triad arrangement of RGBsubpixels. White light emitted from conventional backlight 3 wasutilized in all Examples herein.

The optical construction of Display “A” of this first Example is shownin FIGS. 4-6. NW Display “A” included from the rear forward towardviewer 1, conventional backlight 3, conventional linear polarizer 5 withtransmission axis P_(R), rear transparent glass substrate 17, rearsegmented pixel and RGB subpixel electrodes 21, rear orientation film 7having buffing direction B_(R), liquid crystal layer 9, frontorientation film 11 having buffing direction B_(F), front electrode 23,RGB color filters (not shown) corresponding to each subpixel segment ofelectrode 21, front transparent glass substrate 19, uniaxial positivelybirefringent retardation film 13 having optical axis R, and finallyfront linear polarizer 15 having transmission axis P_(F).

With respect to Display “A”, transmission axis P_(R) of rear polarizer 5was substantially parallel to buffing direction B_(F) of frontorientation film 11. Also, transmission axis P_(F) of front polarizer 15was substantially parallel to rear buffing direction B_(R) oforientation film 7 thus defining an “X-buffed” AMLCD, buffing directionsB_(F) and B_(R) being substantially perpendicular to one another.

Retardation film 13 was positively birefringent and had a retardationvalue of 140 nm. Optical axis R of retardation film 13 was rotated about8.5° in the clockwise direction relative to axis P_(F) and directionB_(R) so as to define Θ as shown in FIGS. 4-5 as about 8.5°. Retardationfilm 13 of Display “A” was obtained from Nitto Corporation, Japan, orNitto Denko America, New Brunswick, N.J., as Model No. NRF140.

Rear and front linear polarizers 5 and 15 of all Examples herein wereconventional in nature and obtained from Nitto Denko America, Model No.G 1220DUN.

FIG. 9 is a contrast ratio contour plot of Display “A” of this Examplewhen a driving voltage of about 6.8 volts was applied thereto and whitelight was emitted from backlight 3. As shown, the high contrast viewingzone was shifted vertically into the positive vertical region (above the0° vertical viewing axis) by the aforesaid clockwise rotation of opticalaxis R of retardation film 13. This display had at least about a 10:1contrast ratio at +10° vertical over a total range of about 110°horizontal, this being an improvement of about 40° with respect to thelight valve of prior art FIG. 1 at the same 10° vertical viewing axis.In a similar manner, Display “A” had at least about a 10:1 contrastratio at +50° vertical that extended over a total of about 95°horizontal, this 95° horizontal range being a signficant improvementover the contrast ratio at 50° vertical with respect to the light valveof FIG. 1.

As shown, the viewing zone or envelope of Display “A” when about 6.8volts was applied thereto was fairly uniform (or unskewed) in nature.Additionally, high contrast ratios (e.g. 50:1) of Display “A” extendedover significantly greater horizontal and vertical expanses than didtheir corresponding ratios in the light valve of prior art FIG. 1. ForExample, the 50:1 contrast ratio of Display “A” at +10° verticalextended over a total of about 80° horizontal as shown in FIG. 9, whilethe corresponding 50:1 contrast ratio curve in prior art FIG. 1 at 10°vertical extended only over about 40° horizontal. Thus, it is evidentthat the addition of retardation film 13 with its correspondingretardation value and optical orientation resulted in a significantimprovement with respect to contrast ratio.

FIG. 10 is a contrast ratio contour plot of Display “A” of this Examplewhen about a 6.0 volt driving voltage was applied thereto. As shown, theslight reduction in driving voltage resulted in the contrast ratiocontours slightly shrinking horizontally in the extreme upper verticalviewing region (e.g. 60° vertical).

FIG. 11 is an intensity (fL) vs driving voltage plot of Display “A”. Asshown, Display “A” had significantly reduced inversion with respect tothat of the prior art light valve shown in FIG. 2. This is evident bythe substantial elimination of the prior art inversion humps present atabout 3.0 volts and greater. No such inversion humps are shown in FIG.11 thus illustrating the significant improvement over the prior art withrespect to inversion at the illustrated horizontal viewing angles alongthe 0° vertical viewing axis. The elimination of the inversion humps ofthe prior art allows Display “A” to be easily and effectively drivenwith a plurality of gray level driving voltages while allowing viewersat different viewing angles to see substantially the same image withrespect to color and other important viewing characteristics.

FIG. 12(a) is a contrast ratio contour plot of NW a-Si TFT driven“X-buffed” Display “B” of this Example, Display “B” being manufacturedand tested for purposes of comparison with Display “A”. As stated above,Display “B” was manufactured utilizing the same liquid crystal material,electrodes, RGB color filters, orientation films, TFTs, and transparentsubstrates as Display “A”. The only difference between Display “A” andDisplay “B” was that the retardation value of uniaxial positivelybirefringent retardation film 13 of Display “B” was about 350 nm insteadof the 140 nm value of Display “A” and optical axis R of retardationfilm 13 was substantially parallel to directions R₀ and B_(R). Thus, bycomparing the results of displays “A” and “B”, one may easily see theimprovement resulting from the use of a retardation value in the rangeof about 100-200 nm (e.g. 140 nm) as opposed to retardation valuesgreater than about 300 nm.

As shown in FIG. 12(a) the high contrast viewing envelope of the 350 nmretardation film Display “B” was significantly smaller with respect tocontrast ratio than was that of Display “A” shown in FIG. 9. Bycomparing FIGS. 9 and 12(a), it is clear that use of the higher valueretardation film resulted in a smaller viewing envelope both verticallyand horizontally.

Further evident from comparing FIGS. 9 and 12(a) is the fact thatDisplay “A” had higher contrast ratios (e.g. 50:1 and greater) over alarger range of viewing angles than did Display “B” thus resulting inimproved viewing characteristics. Thus, this additional advantageassociated with the lower value retardation film is clear.

FIG. 12(b) is a contrast ratio contour plot of NW a-Si TFT drivenDisplay “C” of this Example. Display “C” differed from Display “A” inthat Display “C” was “P-buffed” as shown in FIG. 7 (instead of“X-buffed”) and utilized a uniaxial positively birefringent retardationfilm 13 having a retardation value of about 350 nm. Furthermore, opticalaxis R of retardation film 13 in Display “C” was substantially parallelto axis P_(R) and direction B_(R) (θ=about 0°).

All three NW AMLCDs of this Example had their respective retardationfilms disposed on the forward or viewer side of liquid crystal material9 and sandwiched between front substrate 19 and front polarizer 15.

Display “C”, which is similar to NW displays described in U.S. Pat. No.5,184,236, had its contrast ratio contour plot illustrated when about6.0 volts was applied thereto in FIG. 12(b). As shown in FIG. 12(b) ascompared to FIG. 9, Display “C” had significantly lower contrast ratioexpanses both vertically and horizontally than did Display “A”.Additionally, the extent of higher contrast ratios (e.g. 50:1) inDisplay “A” was greater than that of Display “C” as is evident bycomparing FIG. 9 with FIG. 12(b).

The orientations of retardation film optical axis R in Displays “A”,“B”, and “C”, of course, resulted in the viewing envelopes of Displays“B” and “C” not being shifted vertically as was the envelope of Display“A”.

EXAMPLE 2

A “P-buffed” normally white twisted nematic light valve was manufacturedand tested at about 35°-40° C. in this Example. This light valve hadoptical orientations similar to those shown in FIG. 7 and included fromthe rear forward toward viewer 1 conventional backlight 3, rear linearpolarizer 5 with transmission axis P_(R), rear transparent glasssubstrate 17, rear continuous electrode 21, rear orientation film 7 withbuffing direction B_(R), liquid crystal layer 9 having a thickness orcell gap “d” of about 5.86 μm, front orientation film 11 with buffingdirection B_(F), front continuous electrode 23, front transparent glasssubstrate 19, uniaxial positively birefringent retardation film 13having a retardation value of about 120 nm, and finally front linearpolarizer 15 having transmission axis P_(F).

Retardation film 13 had its optical axis R rotated clockwise about 20°relative to directions R₀ and B_(R) so as to shift the viewing envelopeinto the positive vertical viewing region. In other words, Θ equaledabout 20° as shown in FIG. 7.

Retardation film 13 of this Example was positively birefringent,uniaxial, and was obtained from Nitto Denko America, New Brunswick,N.J., Model No. NRF120. The liquid crystal material was identical to thetype utilized in the displays of Example 1, as were the polyimideorientation films, glass substrates, and polarizers. Because thisExample utilized a light valve, both electrodes 21 and 23 werecontinuous in nature as opposed to the segmented design of the rearelectrode of each AMLCD in Example 1.

FIG. 13 is a contrast ratio contour plot of the NW light valve ofExample 2 when about a 5.0 volt driving voltage was applied thereto. Asshown, the 20° clockwise rotation of optical axis R of retardation film13 resulted in the shifting of the viewing zone or envelope into thepositive vertical region as is evident by FIG. 13. Furthermore, the useof the 120 nm retardation film resulted in high contrast ratios over awide range of horizontal and vertical viewing angles as shown. Thus, theadvantages of such a retardation value and the 20° rotation of opticalaxis R are self-evident in view of the superior viewing characteristicsexhibited.

FIG. 14 is a contrast ratio contour plot of the NW light valve of thisExample when about a 4.0 volt driving voltage was applied. As shown, theviewing zone remained in the upper or positive vertical region and wassubstantially uniform and unskewed in nature.

An advantage of particular interest associated with the light valve ofthis Example is its good contrast at driving voltages of about 4-5volts. Certain driver chips often do not allow displays to be drivenabove 6 volts. In other words, such chips provide for maximum drivingvoltages of only about 6 volts, this meaning that many of the gray leveldriving voltages are around 4-6 volts. Therefore, the superior contrastbehavior of this light valve at such driving voltages is a distinctadvantage. The better behavior of this light valve at lower drivingvoltages is clearly an improvement over the prior art.

EXAMPLE 3

A normally white a-Si TFT driven twisted nematic AMLCD of the P-buffedtype was manufactured and tested at about 35°-40° C. in this Example.The liquid crystal material was the same as discussed above in Examples1 and 2, with this AMLCD having a cell gap “d” of about 5.3 μm in eachof the red, green, and blue subpixels. Each pixel of this AMLCD includedan RGB triad of subpixels. Unlike the other Examples herein, this AMLCDwas driven with a conventional Gross Tester in that all column and rowaddress lines were driven together.

As shown generally in FIG. 7, the AMLCD of Example 3 included from therear forward conventional backlight 3, conventional polarizer 5 havingtransmission axis P_(R), transparent rear glass substrate 17,transparent ITO segmented subpixel or pixel electrodes 21, rearorientation film 7 having buffing direction B_(R), liquid crystal layer9 having a RGB cell gap of about 5.3 μm, front orientation film 11 withbuffing direction B_(F), front continuous electrode 23, red, green, andblue color filters (not shown) corresponding to each subpixel electrodesegment, front transparent glass substrate 19, uniaxial positivelybirefringent retardation film 13 having a retardation value of about 140nm, and finally front linear polarizer 15 having transmission axisP_(F).

Retardation film 13 was again obtained from Nitto Denko America, NewBrunswick, N.J., as Model No. NRF140 and was oriented such that itsoptical axis R was rotated clockwise about 5° relative to directionB_(R) and axis P_(R). In other words, Θ as shown in FIG. 7 was about 5°.

As shown in FIG. 15, the AMLCD of this Example had its viewing zone orenvelope shifted into the positive vertical region by the 5° rotation ofretardation film axis R, the viewing envelope being substantiallyuniform in nature as shown.

This concludes the Examples herein.

As is evident from the results of the aforesaid Examples, the provisionof a retardation film having a retardation value of from about 100-200nm (or 100-250 nm) on a single side of the liquid crystal layersignificantly improves the viewing characteristics of a display withrespect to both constrast ratio and inversion. As will be appreciated bythose of skill in the art, the provision of a normally white twistednematic LCD having an enlarged and vertically shiftable viewing zonewith reduced inversion is a significant improvement over conventionalnormally white LCDs, this improvement allowing the substantially cheaperto manufacture NW displays to take the place of more expensive normallyblack displays.

Furthermore, the ability to shift the viewing zone vertically intoeither the positive or negative vertical viewing region allows themanufacturer to custom make or tailor each AMLCD according to the needsof specific customers. For Example, one customer may require an AMLCD tobe mounted in the lower portion of an avionic cockpit such that thepilot is forever looking downward at the display thus requiring theAMLCD to have high contrast ratios and reduced inversion in the upper orpositive vertical region. In such a case, the desired viewingcharacteristics may be achieved simply by rotation of retardation film13 as discussed above. Thus, the designs of the different embodiments ofthis invention allow different specifications to be realized.

FIG. 16 illustrates another embodiment of this invention where first andsecond biaxial (or uniaxial) retarders are located on the rear side ofthe liquid crystal layer. Such an embodiment is further described andillustrated in U.S. Pat. No. 5,594,568, incorporated herein by referenceabove. Alternatively, the two biaxial retarders may be located onopposite sides of the liquid crystal layer as described and illustratedin U.S. Pat. No. 5,570,214, also incorporated herein by reference above.

The pre-tilt angle of the displays and light valves herein may be about3° in certain embodiments, and the value of “d/p” (thickness/naturalpitch of the liquid crystal material) of the liquid crystal layers maybe set to about 0.25.

Once given the above disclosure, many other features, modifications, andimprovements will become apparent to the skilled artisan. Such otherfeatures, modifications, and improvements are, therefore, considered tobe a part of this invention, the scope of which is to be determined bythe following claims:

We claim:
 1. A normally white liquid crystal display capable ofdisplaying an image to a viewer, the display comprising: a pair ofelectrodes sandwiching a twisted nematic liquid crystal layertherebetween, said pair of electrodes for applying a voltage across saidliquid crystal layer; first and second orientation layers disposedadjacent said liquid crystal layer on opposite sides thereof, said firstand second orientation layers for aligning liquid crystal molecules ofsaid liquid crystal layer in a predetermined manner; first and secondbiaxial retardation members provided in the display; said liquid crystallayer being provided so as to twist at least one wavelength of lightpassing therethrough from about 80°-100°; each of said first and secondbiaxial retardation members at at least one location therein beingdefined by n_(x)>n_(y)>n_(z), wherein n_(x), n_(y), and n_(z) areindices of refraction with the “z” direction being perpendicular to the“x” and “y” directions; and wherein for each of said first and secondbiaxial retardation members d·Δn_(zx) is from about −100 to −200 nm, andwherein said first and second biaxial retardation members are orientedwith respect to each other so that the display can achieve a white lightcontrast ratio of at least about 30:1 over a horizontal viewing angularspan of at least about 80° and over a vertical viewing angular span ofgreater than about 30°.
 2. A normally white liquid crystal displaycomprising: a rear, light entrance polarizer having a transmission axisoriented in a first direction; a front, light exit polarizer having atransmission axis oriented in a second direction wherein said first andsecond directions are oriented so as to define a normally white display;first and second negative biaxial retarders, wherein each of said firstand second negative biaxial retarders has a retardation value d·Δn_(zy)of from about −10 to −100 nm where “d” is the thickness of the retarderand at least a portion of the retarder has indices of refraction n_(x),n_(y), and n_(z) where n_(x) and n_(y) define a plane and n_(z) isoriented in a direction perpendicular to said plane; and wherein saidretarders are so arranged with respect to one another so as to achieve awhite light contrast ratio of at least about 10:1 over a horizontalviewing angular span, at a predetermined vertical viewing angle, of atleast about 120°, and over a vertical viewing angular span of greaterthan about 60° at a predetermined horizontal viewing angle.
 3. Thenormally white display of claim 2, further including a liquid crystallayer having a Δn value of from about 0.075 to 0.095, and wherein thefirst and second negative biaxial retarders are located on the same sideof the liquid crystal layer.
 4. A normally white liquid crystal displayfor displaying an image to a viewer, the liquid crystal displaycomprising: a liquid crystal layer disposed between first and secondelectrodes, said first and second electrodes for applying a voltageacross said liquid crystal layer; said liquid crystal layer beingdisposed between first and second polarizers oriented in a manner suchthat the display is of a normally white type; first and secondnegatively birefringent retardation systems located on opposite sides ofsaid liquid crystal layer so that said liquid crystal layer is disposedbetween said first and second negatively birefringent retardationsystems, each of said first and second negatively birefringentretardation systems having a retardation value of d·Δ_(zy) from about−10 to −100 nm, where the retardation value is defined by a thickness ofthe negatively birefringent retardation system multiplied by adifference between two of three indices of refraction; and wherein saidfirst and second negatively birefringent retardation systems are soarranged with respect to one another so that the display can achieve awhite light contrast ratio of at least about 10:1 over a horizontalviewing angular span of at least about 120° at a vertical viewing angle,and over a vertical viewing angular span of greater than about 60° at ahorizontal viewing angle.
 5. The display of claim 4, wherein each ofsaid first and second negatively birefringent retardation systemsincludes a retarder having three unequal indices of refraction.
 6. Thedisplay of claim 4, wherein each of said first and second negativelybirefringent retardation systems includes a negatively birefringentbiaxial retardation film.
 7. The display of claim 4, wherein saiddisplay is capable of outputting a contrast ratio at least about 30:1 atviewing angles at about +15° vertical over a horizontal range of atleast about 75°.
 8. The display of claim 7, wherein the display iscapable of outputting a contrast ratio of at least about 50:1 over ahorizontal viewing angular span of at least about 80°.